Composites are defined as multiphase materials, which may occur naturally or may be manufactured. Manufactured composites typically are a formulation of one or more materials selected so as to achieve properties that are not individually exhibited by the materials comprising the composite. Composites may be classified based on a type of continuous matrix and dispersed phases, such as a reinforcement. Composite materials comprising at least one constituent phase, primarily the dispersed phase, having at least one dimension on the order of 1-100 nanometers (nm) are referred to as “nanocomposites.” Nanocomposites may be further classified based on category (e.g., organic or inorganic), as well as geometry of nanoscale reinforcement. A few well-known examples of naturally occurring nanocomposites include human bone, seashells, spider silk, and armored fish. As will be appreciated, each of these nanocomposite materials comprises a structural hierarchy (i.e., structure at multiple length scales) which makes them perform exceptionally well as compared with other materials of a similar chemistry.
Material properties of composites are known to be dependent on interactions between the matrix and the dispersed phases. Large surface areas per unit volume at the nanoscale generally cause nanomaterials to function differently than their bulk counterparts. With increased interactions between the matrix and the dispersed phase, nanocomposites are considered relatively superior to conventional composites, providing new advantageous properties without compromising existing beneficial properties, such as strength or durability.
Polyethylene terephthalate (PET) is an aromatic semi-crystalline thermoplastic polyester, first synthesized in the early 1940s. FIG. 1 is a chemical formula illustrating a molecular structure of PET. Polyethylene terephthalate is well known for its strength and toughness, high glass transition and melting points, chemical resistance, and optical properties. Polyethylene terephthalate is commonly used for commodity and engineering applications due to its relatively low cost. Polyethylene terephthalate is characterized by a microstructure wherein longitudinal stretching forms strong fibers with a high molecular chain orientation, as well as bi-axial stretching forming strong films. Linear PET is naturally semi-crystalline. Thermal and mechanical history, such as rate of cooling and stretching, can drive PET to be amorphous or more crystalline, and thus influence its mechanical properties. Although PET is utilized in industries such as fiber, packaging, filtration, and thermoforming, the widespread use of PET generally is constrained due to a slow crystallization rate and a limited barrier performance as compared with other commonly used polyesters.
It will be appreciated that there has been a long felt need for developing lightweight materials for use across a wide range of industries, such as packaging, automotive, and aerospace, and thus attempts have been made to improve material properties through better control of material processing and an addition of reinforcements. For example, increasing the crystallinity of PET improves its mechanical and barrier properties. Restrictions with the material, however, such as crystallization rate, and with industrial processes in maximizing crystallinity, such as cooling rate, cycle time, and stretching process, have limited attempts to improve the material properties of PET. Progress in the field of nanomaterials, however, has led to a development of PET nanocomposites which improve the physical properties of PET, thus making PET more effective for applications within the automotive, aerospace, and protective apparel industries. Different types of nanoreinforcements, such as clay, carbon nanofibers (CNF), carbon nanotubes (CNT), graphene, silicon dioxide (SiO2), and the like, have been found to improve many properties of PET, such as mechanical, thermal, barrier, electrical, fire retardation, optical, surface properties, crystallization kinetics of PET, and the like.
As will be appreciated, exfoliation of nanoreinforcements into individual entities and their uniform dispersion into a polymer matrix is essential for the success of polymer nanocomposites. Uniform in dispersion of nanoreinforcements in polymers may be achieved by way of various approaches, including, but not limited to, melt-compounding, in-situ polymerization, surface treatment of the nanoreinforcements, and the like. Carbon nanomaterials, such as carbon nanofibers, carbon spheres, carbon nanotubes, and graphene, illustrated in FIG. 2, generally are advantageous due to their superior material properties and simple chemistry. Multi-fold property improvements may be achieved through the dispersion of carbon nanomaterials into polymers
Graphene is a relatively new nanomaterial which comprises a single layer of carbon atoms similar to an unzipped single walled carbon nanotube. Single layer graphene generally is twice as effective as carbon nanotubes in reinforcing polymers since graphene has two flat surfaces for polymer interaction whereas a carbon nanotube comprises only one exterior surface for polymer interaction. It will be appreciated that a development of graphene synthesis methods in conjunction with an introduction of new graphene-based nanomaterials, such as graphene oxide, expanded graphite, and graphene nanoplatelets, has made graphene commercially viable. However, application of graphene-based nanomaterials in fabricating polymer nanocomposites has been hindered due to limited information about the influence of graphene nanomaterials in reinforcing polymers.
Melt-compounding and in-situ polymerization have been the most studied techniques for preparing PET-graphene nanocomposites. Although in-situ polymerization is effective in dispersing graphene, the use of in-situ polymerization heretofore has been limited due to difficulties in attaining a desired molecular weight and a need for expensive reactors. Melt-compounding is a straight-forward approach involving shear mixing, but that alone has not been found to be effective in dispersing graphene in several tested polymer systems. As will be appreciated, achieving a homogenous dispersion of graphene nanoplatelets in PET is critical for improving bulk properties. Dispersing graphene in PET is nontrivial, however, as PET generally is highly viscous (500-1000 Pa s) with a melting temperature of 260° C.-280° C. Thus, selecting a process that facilitates working at high temperatures and with highly viscous materials is necessary.
Another important aspect for an implementation of polymer nanocomposite applications is an ability to predict resultant material properties so as to provide flexibility in designing manufacturing processes and reduce developmental costs. Traditional composite models are not accurate in predicting the properties of nanocomposites. Although micromechanical models based on continuum theory have been found to be effective in estimating short fiber composites, few studies have reported an applicability of these models to nanocomposites.
What is needed, therefore, is an effective and reliable process whereby graphene nanoplatelets may be uniformly dispersed in PET so as to provide reinforced bulk PET, and micromechanical models whereby the material properties of reinforced bulk PET may be predicted.